Cordless portable electronic devices, such as such as personal computers, cell phones, and personal digital assistants (PDA), as well as audio-visual electronic devices, such as video camcorders and mini-disc players, are rapidly becoming smaller and lighter in weight. Because these devises are designed to be light weight and compact, a demand for compact and light weight secondary batteries that have a higher energy density than that obtainable by conventional lead-acid batteries, nickel-cadmium storage batteries, or nickel-metal hydride storage batteries has developed.
Non-aqueous electrolyte secondary batteries have been extensively developed to meet this demand. Although lithium is the best candidate for the anode material (3860 mAh/g), repeated dissolution and deposition of lithium during discharging and charging cycles, causes the formation of dendritic lithium on the surface of the lithium. Dendrites decrease charge-discharge efficiency and can pierce the separator and contact the positive electrode, causing a short circuit and unacceptably shortening the life of the battery. In addition, the circuit density is high at the end of a dendrite, which can cause decomposition of the non-aqueous solvent.
To overcome these problems, carbon materials, such as graphite, capable of absorbing and desorbing lithium have been used as the negative electrode active material in lithium non-aqueous electrolyte secondary batteries. When a graphite material is used as the negative electrode active material, lithium is released at an average potential of about 0.2 V. Because this potential is low compared to non-graphite carbon, graphite carbon has been used in applications where high voltage and voltage flatness are desired. However, the search for alternate anode materials is continuing because the theoretical discharge capacity of graphite is only about 372 mAh/g. Thus, these batteries cannot meet the demand for high energy density required for many light weight mobile electrical and electronic devices.
Materials that are capable of absorbing and desorbing lithium and showing high capacity include simple substances such as silicon and tin. Elemental silicon and tin are high energy density materials, and they react with lithium at low voltage with respect to Li/Li+. However, absorption of lithium by silicon or by tin causes the silicon or tin to expand. When the battery case has low strength, such as a prismatic case made of aluminum or iron, or an exterior component which is made of an aluminum foil having a resin film on each face thereof (i.e., an aluminum laminate sheet), the battery thickness increases due to expansion of the negative electrode, such that an instrument comprising the battery could be damaged. In a cylindrical battery using a battery case with high strength, because the separator between a positive electrode and a negative electrode is strongly compressed due to volume expansion of the negative electrode, an electrolyte-depleting region is created between the positive electrode and the negative electrode, thereby making the battery life even shorter. In addition, there is a risk of battery puncture, causing serious safety concerns.
To address these problems, silicon, tin, and silicon/tin composites, with or without carbon, have been proposed as alternate anode materials for lithium secondary batteries. For example, Miyaki, U.S. Pat. Publication 2005/0181276, relates to Co—Sn amorphous composites with carbon for nonaqueous electrolyte secondary batteries. Kawakami, U.S. Pat. Publication 2005/0175901, describes anode materials containing Sn-transition metals and alkali/alkaline earth/p-block element-alloys for non-aqueous secondary batteries. Yamamoto, U.S. Pat. Publication 2005/0084758, relates to carbon coated with Si/Sn anodes for lithium batteries.
However, these materials still have the disadvantage of volume expansion upon incorporation of lithium. They develop cracks and eventually fall off the current collector as the charge/discharge cycle is repeated. Because all the silicon-silicon or the tin-tin bonds are broken when an alloy with maximum lithium content is formed, it is desirable to have anode material having a larger free volume for Li+-ions within the host structure without much change in the host structure when material absorbs lithium. It is also desirable to have inexpensive compound that is also non-polluting, to make the battery both inexpensive and environmentally benign.